.header {
    height: 100px;
    width: 1536px;
    background-color: black;
    opacity: 0.5;
    float: left;
    position: fixed;
    z-index: 3;
  }
  .header img {
    height: 60px;
    width: 250px;
    margin-left: 40px;
  }
  .header input {
    border-radius: 18px;
    outline: none;
    padding-left: 10px;
    background-color: black;
    opacity: 0.7;
    color: white;
    border: 1px white solid;
    height: 40px;
    margin-left: 900px;
    margin-top: 35px;
  }
  .header .submit {
    color: white;
    background-color: transparent;
    border: 1px white solid;
    width: 50px;
    height: 30px;
    margin-left: 30px;
    text-align: left;
  }
  .header .submit:hover {
    color: #4caf50;
  }
  .banner1 {
    width: 100%;
    height: 700px;
    background: url(https://www.tsinghua.edu.cn/qhdxdwzzb/img/bann.jpg) no-repeat
      top center;
    background-size: cover;
  }
  .banner1 h1 {
    color: white;
    font-size: 63.2px;
    text-align: center;
    padding-top: 350px;
  }
  .banner1 h2 {
    color: white;
    text-align: center;
    padding-top: 10px;
  }
  .topnav {
    position: relative;
    top: -30px;
    z-index: 1;
    width: 1200px;
    height: 87px;
    background-color: rgba(189, 19, 19, 10);
  }
  .topnav ul {
    margin-left: 100px;
    width: 1200px;
    height: 87px;
  }
  .topnav ul li {
    display: block;
    float: left;
    font-size: 20px;
    font-weight: 700;
  }
  .topnav ul li a {
    display: block;
    padding: 30px 30px;
    color: white;
  }
  .topnav ul li:hover {
    text-decoration: none;
    background-color: rgb(123, 18, 18);
  }
  .topnav ul li a:hover {
    text-decoration: none;
  }
  .banner2 {
    width: 100%;
    height: 100px;
    background-color: rgb(240, 241, 244);
    position: relative;
    top: -90px;
    z-index: 0;
    float: left;
  }
  .banner3 {
    width: 100%;
    height: 400px;
    margin: 0 auto;
  }
  .banner3 h1 {
    margin-top: -20px;
    text-align: center;
  }
  .banner3 h1 a:hover {
    color: black;
    text-decoration: none;
  }
  .banner3 .hr1 {
    width: 80px;
    height: 1px;
    border: solid;
    margin: 0 auto;
    margin-top: 15px;
    margin-bottom: 30px;
    color: rgb(179, 25, 25);
  }
  .banner3-content {
    width: 60%;
    height: 500px;
    margin: 0 auto;
  }
  .banner3 h3 {
    margin: 20px 0px;
  }
  .banner3 p {
    font-size: 18px;
    text-indent: 2em;
    line-height: 30px;
  }
  .foot {
    position: relative;
    top: 350px;
    width: 100%;
    height: 100px;
  }
  .foot1 {
    position: absolute;
    z-index: 2;
    width: 220px;
    height: 220px;
    background: url(https://www.ecut.edu.cn/_upload/tpl/06/0a/1546/template1546/images/mh1.png);
    background-size: cover;
    left: 70%;
  }
  .foot2 {
    position: absolute;
    left: 13%;
    top: 160px;
    width: 87%;
    height: 230px;
    background-color: rgb(189, 19, 19);
    z-index: 1;
  }
  .foot2 .foot2-content1 {
    width: 250px;
    height: 60px;
    background: url(../images/header.png);
    background-size: cover;
    float: left;
    margin: 60px;
    margin-bottom: 20px;
  }
  .foot .foot2-content2 {
    color: white;
    float: right;
    margin-top: 80px;
    margin-right: 240px;
  }
  .foot .foot2-hr {
    margin-top: 160px;
    margin-left: 60px;
    width: 1030px;
  }
  .foot p {
    margin-top: 25px;
    margin-left: 60px;
    font-size: 5px;
    color: white;
  }
  .foot3 {
    position: absolute;
    left: 10%;
    top: 220px;
    width: 90%;
    height: 170px;
    background-color: rgb(197, 169, 96);
    z-index: 0;
  }
  